1. Field of the Invention
This invention relates generally to a driveline for a motor vehicle, and in particular, to a driveline having a transfer case for directing power to front wheels and rear wheels.
2. Description of the Prior Art
A transfer case may include a planetary gear set for producing either a “high” range, in which the transfer case output is driven at the same speed as the input, or a “low” range, in which the output is driven slower than its input speed. The 4×2 (High), 4×4 (High) and 4×4 Low states of the transfer case are usually selected manually by the vehicle operator by operating a lever or switch. A first position of the lever will cause a range selection device in the transfer case to direct power from the transmission output to a rear drive axle, the 4×2 drive mode. A second position of the lever will cause the transfer case to direct power to both a front drive axle and a rear drive axle, the 4×4 drive mode. The last position will move the transfer case to low range
Conventional rear wheel drive, on-demand transfer case systems use electromechanical actuation of the 4×4 on demand clutch. Length periods to engage the clutch can result in long duration and high speed wheel slip events before the clutch engages and torque is sent to the non-slipping wheels. Also long disengagement periods can interfere with brake traction control.
The high and low ranges are typically achieved by several methods: electric motor, mechanical lever or electro-hydraulically. An example of electro-hydraulic controls is accomplished by alternately engaging and disengaging a hydraulically actuated range clutch. When the 4×4 drive mode is selected, another hydraulic clutch is engaged. The hydraulic clutches that control high and low range operation typically include a clutch pack of alternating spacer plates and friction discs, which are forced into friction contact when a piston located in a cylinder is pressurized with hydraulic fluid, thereby engaging the clutch. The clutch is disengaged by venting the cylinder, which allows a spring to release the piston allowing the plates and discs to separate.
However, even when the discs and plates are disengaged, they are located in close mutual proximity so that the clutch can be quickly reengaged without loss of time required to first move the plates and discs together from a widely separated distance when the operator commands a range change. With the plates and discs closely spaced and the clutch disengaged, hydraulic fluid is continually supplied to the clutch pack in order to cool and lubricate the clutch. In this environment, hydraulic fluid between the discs and plates causes the clutch components to try to rotate due to viscous shear through the thickness of fluid between the plates and discs, even when the clutch is disengaged.
This action produces a continual drag on the powertrain components, increasing fuel consumption and adding to noise and noise amplification in the driveline. It is better to avoid these disadvantages and yet quickly respond to commands to change the selected range.
The multi-plate hydraulic clutch that is engaged when the low range is produced transmits torque that is amplified through operation of a gearset located in the transfer case between the transmission output shaft and the transfer case output. In order to transmit large torque magnitudes, potentially as large as the vehicle skid torque at which the wheels break free from frictional contact with a road surface, the size of the low range clutch is large. Its size presents packaging difficulties in the transfer case where two other clutches, an epicyclic train and a drive mechanism to the front wheels are also located. A solution is required to avoid the packaging difficulties presented by the size of a hydraulically actuated low range clutch.